This invention relates to a photovoltaic (PV) power system and in particular to an improved method and apparatus for converting DC power to AC power for introduction to a utility grid.
The prior art teaches subjecting a photovoltaic array to solar radiation for generating electrical power for a variety of purposes. In U.S. Pat. No. 5,293,447, issued Mar. 8, 1994, to Fanney et al., there is shown a photovoltaic Solar Water Heating System where solar energy is collected and centrally combined where it is converted to electrical energy for heating a water tank.
The prior art also teaches generating electrical power for a utility grid. Photovoltaic arrays produce a DC voltage which is collected and centrally combined and then converted to an AC voltage by a complex power inverter for use on the utility grid. Existing PV power systems differ mainly in the style of PV module mounting. There are passive PV arrays and tracking PV arrays. The tracking arrays have had varying degrees of reliability problems associated with the tracking mechanisms. The problems with passive arrays have been primarily related to the DC connections of the panels.
In U.S. Pat. No. 4,591,965, issued May 27, 1986, to Dickenson, there is shown an inverter for converting direct current (DC) produced by several photovoltaic solar arrays to a polyphase alternating current (AC). Direct current generated by the PV arrays is applied to switching poles which connect the applied direct current to three lines V.sub.A, V.sub.B, and V.sub.C. No two of the arrays supply current to the same AC line and the current generated by each of the arrays is applied to some one of the AC lines as a result of an inverter switching action.
In U.S. Pat. No. 4,922,124 issued May 1, 1990 to Nagataka Seki et al., power converter device comprising a plurality of self-commutated voltage type inverters is disclosed wherein each input of the inverters is connected to a common DC circuit and the outputs of each inverter are connected to designated windings of transformers which have AC windings arranged in a zigzag connection; serially connected AC windings of the transformers are connected to a utility power system via AC switches. Seki et al. teach effective means of preventing DC overvoltage and preventing excess transient voltage when the transformer units are connected to utility power systems by synchronous making. However, the power converter device of Seki et al. is complex requiring control of DC sources which requires resistors for discharging capacitors, and to absorb regenerative energy resulting in reduced efficiency. Seki also requires a DC overvoltage suppression device on AC switch for controlled connection of AC windings, deliberate sequencing of the multiple self-commutated voltage type converters, an auxiliary charging circuit and a particular start-up sequence.
FIG. 1 shows a system block diagram of a photovoltaic power station as known in the prior art. A PV array 12 receives solar radiation from the sun 11 and converts the solar energy into electrical energy in the form of direct current. The DC is collected by DC switch gear 14 and fed to a DC-AC inverter 16 which converts the DC to three-phase AC. The AC is coupled to AC switch gear 18 for control of low voltages and then the AC is fed to a transformer 20 which generates a high voltage three-phase AC. AC switch gear 22 is coupled to the three-phase outputs of the transformer 20 and such three-phase outputs are fed via the switch gear 22 to high voltage transmission lines 24. A key requirement for connection of a PV power station to the utility grid is that the current waveform of the power being delivered have no more than a total harmonic distortion (THD) of 5% with no individual harmonic having a magnitude of greater than 3% with respect to the fundamental current.
In the PV power station as shown in FIG. 1, the single DC to AC inverter 16 represents a single point failure and needs to switch at high frequency, and use pulse width modulation to meet the total harmonic distortion requirements. The DC power for the array 12 is connected via a significant amount of wiring contained generally in underground trunks and then the DC wiring is all combined in DC switch gear 14. A DC voltage has problems associated with plating action experienced when an electrolyte is subjected to a steady DC potential, creating a plating current in the electrolyte. The resulting plating action causes an ever reinforced plated path to ground caused by the material removed from the conductor (wire or connector) by this action. Eventually a ground fault results in the system, and there is additional concern for potential open circuit failure of the wiring or connector as a result of loss of material due to the plating action. An electrolyte is naturally formed with moisture from condensation including rain and materials found in dirt and dust. Rainwater will often be an electrolyte when mixed with air pollutants causing, for example, an electrolyte such as acid rain. In addition, there are many other problems with the present technology as illustrated in FIG. 1 ranging from the DC to AC inverters being complex and very expensive to EMI/RFI issues exacerbated by high frequency switching circuits.
Because PV energy is low density for utility scale power, a power station may cover several acres. Hence, the wire for collection of the power from the PV array is extensive and represents a significant opportunity for the problems described above for DC wiring to occur. All these problems identified with the prior art PV power station are overcome by the present invention.
Further, in the prior art high frequency switching inverters need filters to take care of EMI/RFI and waveform purity. These filters create uncertainties in installations with a utility grid due to Kilo Volt Amp Reactive (KVAR) generation that needs to be provided by the utility; also there are unpredictable interactions with the system that is being connected to, with respect to resonances being formed between the system reactance and the filters. The present invention overcomes the problems and disadvantages of the prior art described hereinbefore.